MOGADISHU, Somalia -- The FBI became involved Thursday in negotiations for the release of the U.S. captain held hostage by pirates off the Somali coast, officials said.

Meanwhile, the U.S.-flagged cargo ship Maersk Alabama headed to port in Mombasa, Kenya, to deliver humanitarian aid the day after it was hijacked, the father of a crew member told CNN.

An 18-person security detail was on board to ensure the safety of the crew during the 50-hour journey to port, the crew said.

FBI negotiators have made contact with the pirates, CNN reported, but it was unclear whether it was direct contact. A naval surveillance plane and a drone are in the area of the lifeboat holding Maersk Alabama Capt. Richard Phillips and four pirates.

The destroyer USS Bainbridge was positioned near the lifeboat.

Pirates who hijacked the vessel Wednesday backed out of an agreement to exchange Phillips for a pirate captured by crew members attempting to retake the ship, Maersk Alabama's second officer Ken Quinn said.

The rest of the 20-member crew was in good shape, Quinn said.

Before going into a Australia-U.S. ministerial meeting in Washington, U.S. Secretary of State Hillary Clinton said: "We are watching this very closely. Apparently the lifeboat has run out of gas."

Phillips "remains hostage but is unharmed," Maersk spokesman Kevin Speers said during a news conference Thursday. "The safe return of the captain is our foremost priority."

The spokesman said the U.S. Navy "is in command of the situation" and that the shipping line is "coordinating with the Navy and all the governmental organizations involved in this crisis."

U.S. Vice President Joe Biden said administration officials were working "around the clock" on the situation.